This is a request for a supplment to an existing grant. The need for this supplement is outlined within th proposal. The objectives for the proposal are essentially identical to the original application. The purpose of this grant application is to study mammalian heart cell function in vitro employing as the major tol of investigation a laser microbeam in combination with various additional methodologies of tissue culture, electron microscopy, electrophysiology, and pharmacology. The specific objectives of the appliiation are (1) to perform a detailed ultrastructural analysis on irradiated mitochondria and cells in various states of laser induced contractility; (2) to investigate the mechanism of laser radiation interaction with the heart cells by performing action spectrum analyses, ultrastructural studies, frequency shifting by second harmonic generation, and thermal analysis; (3) to inquire into the nature of the induced contractility states by employing electrophysiological monitoring of membranes, pharmacological modification of cellullar activity, determination of Ca ions changes, and an inquiry into the pacemaker and non-pacemaker activity of cells.